An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths to a point or small area of convergence near the tube screen. In one type of inline electron gun, shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lens for focusing the electron beams is formed between two electrodes, referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having their bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focusing lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29-mm internal diameter tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focusing lens design. It is known in the art that, the larger the focusing lens diameter, the less will be the spherical aberration which restricts the focusing quality.
In addition to the focusing lens diameter, the spacing between focusing lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens, which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.
In U.S. Pat. No. 4,370,592, issued to R. H. Hughes and B. G. Marks on Jan. 25, 1983, an electron gun is described wherein the main focusing lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein, equal to the number of electron beams, and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focusing lens is to provide the gentle voltage gradient sought to reduce spherical aberration. Because of the asymmetrical shape of the peripheral rims of the two electrodes described in the patent, horizontal and vertical focus voltage components for the inner and outer guns are not the same. In the vertical direction, the center electron beam sees more of a slot and experiences more focusing action than do the side electron beams, whereat the focusing geometry is bounded, in part, by a circular arc. This is because the field penetrates the slot more easily than an inscribed circular boundary in the vertical direction. Likewise, the horizontal focusing component at the outer electron beams may be more active than at the center beam, because the field in the horizontal direction falls away more rapidly at the sides of the peripheral rims than within the center of the recess.
Although the above-cited patents disclose electron gun designs having decidedly improved performance over prior inline electron guns, it is desirable to further improve such gun designs to reduce the differential in strengths between the horizontal and vertical focusing fields and to enlarge the recesses in their focusing electrodes, thereby enlarging their main focusing lenses. Such enlargement of the recesses, however, is greatly limited by the design of the above-mentioned electron guns because of their use of two or four electrode support rods or beads. The present invention provides an electron gun structure having more uniform relative strengths between the horizontal and vertical focusing fields, and which provides an enlarged focus lens, utilizing standard support rods.